Composite materials

ABSTRACT

A laminated composite material having patterned conductors integral to its structure made of several cloth layers and a penetrating resin matrix material in which at least one of the cloth layers has a deposited patterned layer of electrical conductor to form a conducting path in the resultant composite material. The electrical conductor is preferably deposited to such a thickness that individual fibers of the cloth are coated but that the resin permeates between individual fibers in the conducting regions. The conducting regions provide for external connection of embedded electronic devices.

The invention relates to composite materials having patterned conductorsintegral to their structure, and to composite materials comprisingembedded electronic devices utilizing such patterned conductors.

In many modern structures use is made of fibre polymeric matrixcomposite materials and in particular fibre resin composite materials.These are frequently fabricated from one or more cloth layers of afibre, such as glass or carbon fibres, formed into a woven fabric or amat, together with some permeating polymeric matrix. Considerablepotential utility has been seen in the embedding electronic deviceswithin such materials. Such devices might include strain gauges,temperature sensors and similar sensing devices, embedded identificationtags, and also aerials and the like.

Problems are encountered however in effecting electrical contact betweenembedded devices and the outside world. Structures are known whichemploy fine conductors such as thin wires which trail through thelaminate to an edge. Such connections present potential points ofweakness at the conductor matrix interface, which is prone toseparation, have a tendency to break easily during fabrication, and onlyallow a connection to be made at an edge of the composite laminate. Thewires take an essentially arbitrary path which allows the possibility ofshort-circuits in multiwire systems. The position of wires at the edgeof the panel is also difficult to control, making the use of multi plugconnectors virtually impossible. A further problem in that has beenencountered is that the wires tend to break at the edge of the panelduring autoclaving. It is clear that the trailed wire technique haslimited application to embedded devices.

In more conventional situations multiple electronic devices are mountedon a printed circuit board, a board coated with a patterned layer ofelectrical conductor, which much simplifies the task of connecting alarge number of devices. The principle can be applied to produce printedcircuits either on rigid substrates or in a flexible form by coating theconductor layer onto a thin polymeric substrate. Neither type ofconventional printed circuit board, however, is readily suitable forincorporation into laminated composites as they would represent asignificant plane of weakness in the composite structure exhibiting atendency to delamination at the interface with the resin matrix. Printedcircuits comprising a conductive pattern on reticulated or foraminousbases are known from, for example, U.S. Pat. No. 3,053,929, but theconducting strip so formed in the conducting regions of the patternremains a potential zone of weakness producing a tendency todelamination.

It is an object of the invention to provide a composite materialincorporating a connection system with the case of connection and use ofprinted circuits which is more compatible with use in laminatedcomposites.

According to an aspect of the invention there is provided a compositeprepreg comprising a cloth layer formed from fibres of a materialsuitable for incorporation into a fibre and polymeric matrix compositehaving deposited thereon a patterned layer of electrical conductor, theelectrical conductor being deposited to such a thickness that individualfibres of the cloth are coated but that permeation of the cloth by resinremains possible in the conducting regions, and the cloth beingimpregnated with an uncured, curable polymeric composite matrixmaterial.

The invention enables complex patterns of conductors, analogous to thosewhich can be produced on printed circuit boards, to be laid down ontothe cloth allowing ready connection of a plurality of devices in aformat which is suitable for incorporation into a resin matrixcomposite. Such complex and precisely ordered patterns could notpracticably be incorporated using wires.

The cloth material may comprise woven or laid fibres or an otherwisepartially porous web-like structure, provided that it is susceptible topenetration by matrix material. Since matrix material is able topenetrate between the fibres of the cloth in the unpatterned areasduring composite production, the tendency to delamination at the clothlayer is limited and the material retains good through thicknessstrength which could not be obtained by incorporating conventionalprinted circuits. Better integrity and even lower tendency todelamination when the cloth is incorporated into a composite material isobtained because conducting material in the conducting regions of thepattern is laid down to a thickness where a consistent coating of theindividual cloth fibres, and thus a conduction path, is produced but asolid conducting strip is not formed and the cloth in the conductingregions of the pattern remains at least partly an open mesh which willallow penetration of a matrix material between the fibres. Theelectrical conductor is thus deposited to such a thickness thatindividual fibres of the cloth are coated but that permeation of thecloth by the resin remains possible in the conducting regions. The resinis then able to penetrate between the fiber during composite production,so that the conduction paths become integral to the structure and theirtendency to provide delamination sites is reduced.

Applying metal to a preformed cloth layer, rather than attempting toweave conductors into a cloth layer, ensures that adjacent fibres areprovided with consistent, continuous metal coatings in the conductingregion. This ensures that each fibre is connected to those surroundingit by a multitude of different pathways, and minimizes the risk of shortcircuits occurring in the region of a patterned track. This improvedreality allows thinner tracks and obviates the need for redundanttracks, so that thinner, higher density conducting tracks can beproduced allowing for the embedding of complex, multiply-connecteddevices such as microprocessors.

According to a further aspect of the invention there is provided alaminated composite material comprising a plurality of cloth layers anda penetrating polymeric matrix material, such as a resin matrixmaterial, wherein at least one of the cloth layers has deposited thereona patterned layer of electrical conductor, the electrical conductorbeing deposited to such a thickness that individual fibres of the clothare coated but that the matrix material permeates therebetween.

This produces a composite panel having integral conductors with all theadvantages of complexity and structural integrity described above. Inaddition, connection can be made at any point in a conductor path via ahole through the other layers and is not restricted to an edge of thelaminate as is the case where wires are used.

The invention is particularly applicable to laminated multilayer clothand polymeric matrix composite materials, and in particular resin matrixcomposite materials, in which a conductive layer is laid down inaccordance with the invention onto one or more of the cloth layers. Itwill be understood however that the invention is not restricted to suchmaterials but encompasses composite materials wherein at least oneconducting cloth layer is incorporated into a polymeric matrix, with orwithout further reinforcing materials in the polymeric matrix of cloth,fibrous or other form, and whether in isolation or in combination withfurther known components selected for their structural or otherproperties.

The invention provides conducting paths which may be used in connectionwith one or more embedded electronic devices electrically connected tothe patterned layer to effect external electrical connection to andinternal connection between the embedded devices. Examples of deviceswhich can usefully be incorporated into the laminate include sensorssuch as strain gauges and actuators such as shape modifiers. Theinvention also allows devices such as aerials, resonant structures andfrequency filters to be formed directly by selecting an appropriatepattern for the conducting material. The invention is also useful in theformation of devices to monitor the degradation of the matrix material,for example by building into the structure capacitive devices which arecapable of detecting alteration in the dielectric behavior of the matrixmaterial, such as could be caused by mechanical loading, waterabsorption etc. within the composite. Patterns can be selected forproperties other than electrical resistivity, for example to give theresulting laminate particular magnetic properties.

Furthermore, the conducting paths may be used to form inductive or othersimilar structures capable of interacting with suitably tuned systemsexternal to the laminate. In this embodiment a laminated compositematerial is formed in which at least some of the conducting paths areconfigured to form one or more resonant structures suitable foreffecting remote connection between an embedded device and a suitablytuned system external to the laminate. In combination with embeddeddevices it would then be possible to dispense with direct connectionsaltogether and enable the devices to be inductively coupled to theoutside world, further limiting the likelihood of mechanical degradationand mechanical weakness arising from inclusion of the cloth layer.

A particular application of this coupling lies in the use of embeddedidentification devices. An identification chip may be incorporated intothe panel and provided with a suitable coupling to allow interrogationfor identification purposes. Since the chip is embedded and isolated itis largely tamper proof. Such ready interrogation of panels will beuseful in constructing or surveying large structures.

A particular application of the laminate is in production of smarttuneable electromagnetic corner reflectors especially for use withenhanced vision system (EVS) by civil aircraft.

In radar practice it is well known to employ corner reflectorscomprising two or three flat plate reflectors (referred to as dihedralor trihedral reflectors respectively). By maintaining an orthogonalrelationship between the plates, reflection in the reverse direction toincidence is achieved at a wide range of angles of incidence. The returncorresponds to the main specular lobes for the individual plates, sothat a high profile return reflection is provided from a range ofdirections for applications where enhanced radar cross section isrequired such as detection beacons on shipping (see for example "RadarCross Section". Knott, Shaeffer and Tuley, Artech House Inc 1985, pp175-177).

The diffraction effects discussed above require that the angles betweenthe plates are maintained close to 90° to maintain reflectivity.Considering, for example, a dihedral reflector with the angle betweenthe plates increased to π/2+δ, where δ is small. Geometrical opticsdictates that the direction of the reflected beam will differ from thereverse of the incident beam by 2δ. Should this discrepancy exactlymatch the half width of the reflected lobe, the observer at the incidentsource will be situated in a null. Further opening out will move himinto the first side-lobe and there will be an alternation of nulls anddiminishing peaks as δ is increased. Thus the effectiveness of thedihedral depends fundamentally on the interplay of the angular error δand the semi lobe width of order 2λ/d. The physical principle of thetrihedral is the same as for the dihedral and a similar workingparameter can be used.

An analysis of bistatic scattering from flat plates coupled withreflections at a distorted dihedral shows that for symmetrical incidencenulls occur at δ=(X/√2)(λ/d) for integer values of X. Thus smalldeviations from the orthogonal can produce rapid decrease in reflection,especially at the millimetric end of the RF range: for example with anot untypical 100 mm dihedral at 94 GHz the reflection profile willreduce to a first null at 1.30°. To maintain the large reflectionprofile which corner reflectors are designed to provide angels mustclosely approximate to orthogonal.

The embodiment of the invention applies this feature of scatteringbehaviour to produce a corner reflector comprising at least two flatreflector plates, each comprising a composite material as hereinbeforedescribed providing electrical connection to at least one actuator, theat least one actuator comprises means capable of varying the internalangle between the reflector plates between a first angle at which thereflectance of electromagnetic radiation is substantially at a peak anda second angle at which the reflectance of electromagnetic radiation issubstantially at a null.

Appropriate angles for peak and null reflectance are selected dependenton incident radiation wavelength and plate dimensions as discussedabove. The reflector can then be in effect switched on and off to aparticular frequency of incident radiation by use of the actuation meansto switch from the configuration where reflectance is at a local maximumand a large response signal is obtained to the configuration wherereflectance is at a local minimum and a much smaller response signal isproduced. It is desirable to maximize the difference between the peakand the null reflectances, and to achieve this the first angle ispreferably set substantially to 90° to utilize the broad maximum inreflectance cross section obtained from orthogonally arranged plates.The second angle is then conveniently selected to be at a deviation from90° which corresponds to the first null in reflectance.

There actuator preferably comprises an actuator material which exhibitsa strain response to an electrical actuation signal transmitted throughthe conducting laminate, such as is shown by magnetostrictive orelectrostrictive ceramics and piezoelectric materials. Use of a laminatein accordance with the invention enables the actuator material to beincorporated integrally into a composite laminate reflector plate,obviating the need for external moving parts.

Of these materials the magnetostrictive materials are unlikely to bepreferred for most applications of the invention due to theirrestrictive size. For these reasons, electrostrictive and piezoelectricmaterials are likely to be preferred choices for the actuator material.Electrostrictive materials can produce more useable strain thanpiezoelectric materials (500 με compared with 350 με) and demonstrate areduction in hysteresis. However, they show a distinct variation inperformance with temperature. This last point could be overcome withsensitive strain monitoring techniques and closed loop control.Nevertheless, for simplicity of operation the most preferred actuatormaterial will be a piezoelectric material such as PZT. This can producean acceptable maximum strain of 350 με with good bandwidth performanceand demonstrates a relatively good tolerance to variations intemperature. However, it is susceptible to creep and shows a higherlevel of hysteresis.

For accurate control of the active plates creating the corner reflectorsome form of strain monitoring system will be required. This will allowsuch effects as actuator hysteresis and creep to be compensated for withtime. Many different types of strain sensors are available. Forintegration into the composite plates preferred options include PVDFpolymer, fibre optic cables or simply a resistance strain gauge. Again,the conducting paths in the cloth allow direct connection to embeddeddevices within a composite panel.

A number of suitable methods can be used for laying down intricatepatterned conductors onto the cloth. Patterns may simply be screenprinted with conducting inks such as silver loaded inks. Alternatively,electroless and/or electrochemical deposition processes may be coupledwith a suitable patterning. A possible approach involves aphoto-lithographic patterning technique in which first coating auniformly premetallized cloth is first coated with a photoresist whichis exposed in the desired pattern and developed to reveal themetallization in the patterned areas, then thickening up themetallization in the patterned areas for example by electrodepositingfurther metal, and finally stripping clear the remaining photoresist andgiving the cloth a further etch sufficient to remove the metallizationin the unpatterned areas leaving a patterned conducting layercorresponding to the thickened areas. In a modification to this approachan appropriate pattern is printed onto a premetallized substrate byscreen printing with a non-conductive ink, such as common printers ink.Once dry, the ink can serve the same purpose as the photoresist in theprevious example.

The invention will now be described by way of example, with reference tothe accompanying drawings in which:

FIGS. 1 and 2 illustrate a laminated composite panel incorporatingstrain gauges within the composite and electrical connections formedusing patterned cloth according to the invention;

FIG. 3 illustrates a device provided with an inductive coupling formedusing patterned cloth according to the invention;

FIGS. 4 and 5 illustrate degradation monitors formed within a laminatedcomposite panel using patterned cloth according to the invention.

FIGS. 6 to 11 illustrate a laminate according to the invention used in atuneable electromagnetic corner reflector.

The patterned cloth in the examples of FIGS. 1 to 5 is prepared via aprocess based on photolithographic patterning. Woven polyester cloth isfirst metallised by an electroless chemical deposition process, which isa well established technique for producing commercially availablemetallised cloth having a covering of copper, copper on nickel, iron,gold, silver, and other metals susceptible of deposition in thisfashion. In the examples prepared here the technique is used to put downan essentially uniform layer of copper or of copper on nickel.

The metallised cloth is coated with a photoresist, resistant to chemicalattack, which is photolithographically patterned with the desiredpattern by selective exposure to ultraviolet light and immersion in analkali developer solution in such a way as to reveal only those areas ofthe metallized cloth which it is intended will ultimately carry thedesired metallised pattern. The metal coating of these areas is thenthickened by electrodeposition of further copper. After this theremaining photoresist is wholly removed by immersion in an organicstripping solution, and the cloth subjected to a light etch in ferricchloride or ammonium persulphate which removes the unthickenedmetallization completely but leaves metallization in the thickenedpatterned regions. To produce the optimum integral strength when thecloth is incorporated into a composite material the process ofthickening by electrodeposition is carefully controlled to ensure thatmetal is laid to an appropriate thickness so that after the light etch aconsistent coating of the individual cloth fibres is produced but theconducting regions of the pattern remain penetrable by a matrix resin.

FIG. 1 illustrates the cloth after this treatment. Strip resistancestrain gauges 2 are located in a standard 120° array on the cloth 4 togive optimum strain measurement capability in two dimensions. A patternof conducting paths 6 is laid out using the above described method toprovide electrical connection to the strain gauges.

FIG. 2 shows the cloth 4 is incorporated into a multilayer laminatedcomposite panel 10 comprising further layers of woven polyester cloth inan epoxy resin matrix, as is illustrated in FIG. 2. Holes 12 are drilledthrough the material to the conductors, whose path is illustrated by thebroken lines 14. External connection to the strain gauge array can thenbe made through the holes 12, obviating the need which arises with wireconductors to make connection at the edge of the composite panel 10.

FIG. 3 illustrates an alternative procedure for utilizing the patterningtechnique to effect an electrical connection to an embedded device. Aninductive loop of conductive material 24 is fabricated onto a cloth andthe cloth layer incorporated into a multilayer laminated composite panel21 in like manner to the above. The device 22, which can be any suitablesensor, actuator or the like, is embedded in the panel 21 andelectrically connected to the loop 24. The device can then beinterrogated via inductive coupling with an external interrogator loop26. The general principle will be familiar to those skilled in the art,but use of the patterned conducting cloth facilitates fabrication andincorporation into the laminate.

FIG. 4 illustrates use of the patterning principle to form a deviceitself within the laminate. Two conducting layer 31 with connectingpaths are fabricated onto cloth as above and incorporated into amultilayer laminated composite panel 32 so as to lie parallel to eachother, connection being effected via wires 33 to a monitor 34. The stateof the resin matrix between the layers can be monitored, and corrosion,delamination, propagating cracks and the like detected, by using themonitor 34 to detect changes in the resistance or capacitance betweenthe two embedded layers. Incorporation of a plurality of such devicesthroughout a structure would enable early warning to be given of boththe existence and the location of effects.

Both the above principles are combined in the device illustrated in FIG.5. The degradation monitor comprises two C-shaped conduction regions 41embedded in the laminated composite 42 so as to lie opposite each otherin parallel planes 43. This configuration allows direct interrogation ofthe degradation monitor via inductive coupling through an appropriatelytuned external device 44, obviating the need for direct electricalconnection.

FIGS. 6 to 11 relate to a particular embodiment of the inventioncomprising a tuneable electromagnetic corner reflector. It will beunderstood, however, that those features of the embodiment relating tothe production of patterned cloth, external connection, and embedding ofstrain gauges will be of general applicability and not restricted tothis particular application of the invention.

The base material for the fabrication of the patterned metallic clothsis a polyester woven cloth coated with a thin layer of copper. Thecopper is electrolessly deposited and hence each of the individualfibres is electrically connected to those adjoining it. This makes itideal for its usual use as a electromagnetic shielding material but alsoallows the metallization to be patterned (not necessarily parallel tothe weave of the cloth) into continuous tracks to produce material inaccordance with the invention. Two fabrication methods have beendeveloped. These are based on the selective etching and electroplatingof areas defined by photolithographic techniques.

The initial steps in both the selective etching and electroplatingmethods are essentially the same, i.e. selected areas of the cloth mustbe coated with a material that will inhibit the action of the subsequentprocesses on that particular area.

The cloth is first stretched to produce a flat surface without creasesor folds. In the initial studies this was achieved using a 50 cm squareframe to which the cloth was attached by adhesive. The stretched clothwas then coated with a liquid photosensitive emulsion and allowed to dryfor 24 hours to form an impervious layer over the cloth. The emulsioncan however be patterned by selective exposure with ultra-violet lightand development in an alkaline solution. In this example the emulsion ispositive working, that is areas which are exposed are removed, butnegative working resists are readily available and their use will befamiliar. Positive resist was chosen due to its ease of application andprocessing and improved resolution compared to negative equivalents.

The conductor pattern required on the cloth is generated in a CADpackage. The pattern is then transferred to a UV opaque emulsion on anultra-violet transparent film. This transparency acts as the photomaskfor selective exposure of the pattern into the photosensitive emulsionon the copper coated cloth.

The photomask is then placed in intimate contact with the surface of thecloth emulsion and exposed with ultra-violet light. This is a similarprocess to that used in microelectronics industry although thetolerances and dimensions in this case allow the use of a light boxrather than a high precision mask aligner. The cloth is then washed withan alkaline solution to remove the areas exposed to the UV and hence touncover the underlying copper. The minimum feature size that could bedefined using this particular equipment is approximately 250 μm. Thefeature size will be limited by both the exposure equipment, emulsionuniformity/sensitivity and the weave of the cloth substrate.

The first of the two alternative techniques used to produce thepatterned cloths involves the selective electroplating of these exposedregions using the electroless copper as a plating bridge. The patternedcloth is simply used as the anode in a standard electrodepositionarrangement using a acidified copper sulphate solution (100 gm/litre)and a sacrificial copper cathode. A plating current of approximately 40mA per mm² of linear cloth area is used. This is a slightly highercurrent density than normally used for plating flat surfaces due mainlyto the additional area contributed by the cylindrical woven strands. Thecopper is plated to a thickness of approximately 100 μm. After platingthe photosensitive emulsion was removed from the cloth followed by athorough rinse in distilled water to remove any residues.

The cloth is then placed in a weak solution of ammoniumperoxodisulphate. This solution slowly etches copper from the whole areaof the cloth. The thinner unplated areas are completely removed withinapproximately two minutes leaving metallization present only in theisolated patterned areas of thicker electoplated copper.

The second technique relies solely on etching of the areas of copperunprotected by the patterned photosensitive emulsion. After patterning,the cloth is simply placed in the copper etchant described above untilthe exposed areas of copper are removed. The emulsion can then beremoved to again leave the patterned copper.

The essential difference between the two techniques is that in theelectroplated case the holes in the resist take the shape of the coppertracks and in the etched case the emulsion left after patterning takesthe path of the tracks. This can be best shown through consideration ofthe relevant photomasks shown in FIG. 6. Both techniques produced thedesired results, i.e. a patterned cloth, but each has its relativeadvantages and disadvantages.

The electroplated cloth is perhaps the more reliable since it enhancesthe connection between adjacent woven strands through the formation ofplated bridges. These bridges ensure the production of a conductingtrack and their low resistance means they can handle a larger electricalcurrent flow. The thicknened copper does however introduce a largerinterlaminar defect into the composite.

The etched cloth has a minimal thickness of copper and hence has alimited affect on the mechanical properties when embedded in a compositematerial. The reliance solely upon the electroless copper to produce theelectrical path through the cloth does however mean that somediscontinuities can occur, especially in the thinner track widths (<300μm). These can be catered for by building in some redundancy in thecircuit design. In the fabrication of the etched cloths great care mustbe taken to fully expose and to remove all of the emulsion in theregions between the tracks. Failure to do this can produce shortcircuits between separate devices.

As described earlier the conventional technique for taking electricalconnections from an active device embedded in a composite panel is tofeed a wire from between two of the laminates to the edge of thematerial. The inclusion of a patterned cloth carrying the equivalent ofa printed circuit equivalent within the composite means the tracks canbe accurately terminated, in an ordered fashion, at a predeterminedposition within the material. This permits the use of a more controlledand predictable connection technology.

When embedding the cloth substrate its position in the laminate is notedwith reference to a point on the panel. If the panel is thin or is madefrom an optically transparent resin system the track terminations can beeasily located by placing the panel in front of a strong light source.If the panel is optically opaque the terminations can be located byplacing a template, aligned in the same position as the cloth in thepanel, over the external surface of the panel.

An example of a suitable connection is illustrated in FIG. 7. Once thetrack terminations are located small diameter holes (<500 μm) 71 aredrilled through the front face of the laminate 72 to the depth of thecloth 73 at each point where a connection is required to a conductingtrack 74. The use of the plated cloths helps in this procedure sincemetallic particles can be seen in the swarf when the copper track isreached.

Electrical connection may be made by placing a small amount ofconductive epoxy into the hole and inserting a thick wire. The panel maythen be placed in a low temperature oven to cure the epoxy and producethe electrical contact. This procedure is greatly enhanced if the trackseparation is chosen to match the pitch of a standard multi-plugconnector such as is illustrated in FIG. 7. The array of holes 71 aredrilled to match with the connector pins 75 of the multi-plug connector76. The connector can be given some additional support by securing it tothe panel in a non-conducting epoxy.

The drilling of holes through a composite structure will locally degradethe mechanical performance. This disadvantage must however be consideredin the context of the connections to a number of devices, spread over anarea of perhaps 10 m², all being taken from a single connector which canbe placed at a convenient position of minimum structural demands.

For example and testing purposes, two cloth circuits with connected tosmall (3×5 mm) polyimide film strain gauges were embedded in a compositesample fabricated from glass fibre cloth pre-impregnated with epoxyresin. The strain gauges were connected to the conducting tracks usingcomposite epoxy. The cloths were arranged so that the strain gaugesoverlayed each other in the panel and external electrical connectionsmade using the techniques described above. In this particular case, tominimise the mechanical effects on the relatively small substrate, onlypins were placed in the drilled holes rather than full multi-pinconnectors. A third external strain gauge was attached, over theembedded gauges, onto the surface of the composite.

The test sample was then placed in a mechanical test rig and the outputsfrom each of the strain gauges monitored as varying strain levels wereapplied. The outputs from each of the strain gauges are shown in FIG. 8.

It can be seen that each of the strain gauges give comparablemeasurements. This indicates that valid results can be obtained fromstrain gauges embedded in a composite using the metallised cloths.

A further example of the invention is illustrated in FIG. 9, in which a250×250 mm plate containing a number of separate piezoelectric elementsis shown. The piezoelectric elements 93 are made from PZT5H which is astandard piezoelectric ceramic with plain electrodes. Each of theelements was 100×5×0.25 mm. Two metallised cloths 91 were prepared witha number of tacks 92, connected in parallel, running across the diagonalof a square of side 200 mm. These cloths were used to supply highvoltages to the PZT.

The cloths 91 were embedded in a panel constructed from pre-impregnatedglass fibre material 94, 96 and provided with copper conducting tracks92 as described. The PZT elements 93 were attached to the copper tracksusing a continuous bondline of conductive epoxy. In previous studiesthis method of attachment has been shown to produce the greatestdeflections. The pre-impregnated laminate 96 that lays between the twocloths was cut so that it fitted around the PZT elements. This layerreduces the possibility of short circuits or high voltage breakdownbetween the powered cloths. Electrical connections were made using asimple two pin connector 98 to each of the cloth layers in similarfashion to the connector illustrated in FIG. 7.

The use of the printed cloths in this embodiment has considerablesimplified the construction procedures used previously. The examplegiven is a simple illustration of the use of the conductive patternedcloth. It can be seen that the technology could more fully be exploitedby construction of a similar panel using interdigitated PZT (to increaseavailable strain) together with integrated strain gauge monitoring. Theuse of interdigitated PZT entails a doubling in the number of electrodesand hence a significant increase in the complexity of the actuatedplate. Using conventional techniques this would present a challengingfabrication exercise, however using the patterned cloths only a simplechange to the pattern design is required.

FIG. 10 illustrates a trihedral corner reflector comprising threemovable plates 101 of glass reinforced polymer composite with integralPZT piezoelectric actuators. The reflector is in the orthogonal positionfor maximum radar cross-section. The broken lines 102 illustrate thepositions the plates 101 will occupy following actuation of the PZTactuators to cause bending of the active plates to a null-reflectingconfiguration. It can be noted that in the latter configuration theplates are no longer planar as a consequence of the deformationcharacteristics of the actuator material, which produces movement inbending and extension, but this is not found to have a significantadverse effect on the operation of the reflector according to theprinciples outlined above.

A particular application of the invention is in the field of runwayidentification systems to aid landing of aircraft in poor visibility.During recent years there have been many incidents of commercialairliners becoming disorientated when approaching an airfield throughthe use of their Instrument Landing System (ILS) and attempting to landon taxiways and even perimeter roads. This is a particular problem atcertain modern high capacity airports which operate with multipleparallel runways. This has led to a desire for the development of activeor passive millimetric radar system for civil aircraft known as EnhancedVision systems (EVS). Such equipment would use either 35 or 94 GHzfrequencies and a Head Up Display (HUD) to create a visual image of theairport and runway. The pilot would then be able to fly the aircraftdown the approach glide path through extremely poor weather conditionswithout having to rely on the ILS.

A series of dihedral corner reflectors 111 are positioned along thelength of a runway 15 (FIG. 11). The system could equally incorporatetrihedral reflectors. With the plates of the corner reflector adjustedto an orthogonal position the reflector exhibits a high radar crosssection. An incoming aircraft 112 with a radar transceiver in the nose113 receives strongly reflected signals returned along the paths 114producing an enhanced image suitable for producing the head up displaysdiscussed above. Operation of the actuation mechanism to producedivergence of the plates allows them to be switched on or off and evenadjusted to counter any variation in approach angle of the incomingaircraft. Clearly it would be an advantage if only the operationalrunway was identified as too much information may make interpretation ofthe image difficult. This is particularly the case at high volumeairports with more than one parallel runway operating sequentially. Useof the invention in a runway identification system as described allowssuch selective identification with passive reflectors by allowingselective actuation of the reflectors.

At the millimetric wavelengths under consideration for civil aircraftEVS systems manageable deflections achieve the first null withoutrequiring excessively sized plates. For example with a 100 mm dihedralthe reflection profile will reduce to a first null at a deflection of3.45° at 35 GHz and at a deflection of 1.30° at 94 GHz. A furtherexample of a use for the switchable corner reflector is to provide apassive digital RF signalling device. The reflector is switched betweenits orthogonal "on" configuration and its "off" configuration in apredetermined pattern in accordance with a suitable message relayingsystem (such as Morse code). The message is thus visible only to aremote observer who interrogates the reflector with a radar signal ofthe appropriate frequency, but is not transmitted as such.

Other uses of the invention will suggest themselves to those skilled inthe art of radar signalling and reflection.

We claim:
 1. A laminated composite material comprising a plurality ofcloth layers having deposited thereon a patterned layer of electricalconductor, the electrical conductor being deposited to such a thicknessthat individual fibers of the cloth are coated but that permeation ofthe cloth by resin remains possible in the conducting regions, the clothbeing impregnated with an uncured, curable polymeric composite matrixmaterial, the laminated composite further comprising at least oneembedded electronic device electrically connected to the patterned layersuch that the patterned layer effects external electrical connection tothe embedded device.
 2. A laminated composite material according toclaim 1 comprising a plurality of embedded electronic deviceselectrically connected to the patterned layer such that the patternedlayer effects internal connection between the embedded devices.
 3. Alaminated composite material according to claim 1 wherein at least oneembedded device is a sensor.
 4. A laminated composite material accordingto claim 1 wherein at least one embedded device is an actuator.
 5. Alaminated composite material according to claim 1 wherein at least someof the conducting paths are configured to form one or more resonantstructures for effecting remote connection between an embedded deviceand a tuned system external to the laminate.
 6. A corner reflectorcomprising at least two flat reflector plates comprising laminatedcomposite material according to claim 7 wherein the at least oneactuator comprises means capable of varying the internal angle betweenthe reflector plates between a first angle at which the reflectance ofelectromagnetic radiation is substantially at a peak and a second angleat which the reflectance of electromagnetic radiation is substantiallyzero.
 7. A corner reflector according to claim 6 wherein the first angleis substantially 90°.
 8. A corner reflector according to claim 7 whereinthe second angle is at a deviation from 90° which corresponds to thefirst fix in reflectance.
 9. A corner reflector according to claim 6wherein the at least one actuator comprises a piezoelectric materialembedded within the laminated composite material.
 10. A method ofmanufacture of a cloth layer according to claim 1 comprising screenprinting with conducting inks to produce a layer of electricalconductor.
 11. A method of manufacture of a cloth layer according toclaim 1 comprising photolithography to produce a layer of electricalconductor.
 12. A method according to claim 11 comprising the steps of:coating a premetallized cloth with a photoresist; exposing thephotoresist in the desired pattern and etching to expose the patternedareas; thickening and the metallization in the patterned areas;stripping clear the remaining photoresist; giving the cloth a furtheretchant sufficient to remove the metallization in the unpatterned areasleaving a patterned conducting layer corresponding to the thickenedareas.